The present invention relates generally to the field of electromagnetic clutches, and more particularly, is directed to an improved engagement device for an electromagnetic spring-wound clutch.
Electromagnetic spring-wound clutches which may be used to couple a compressor to a prime mover are well known. For example, U.S. Pat. No. 3,735,847 to Brucken discloses the basic construction of an electromagnetic spring-wound clutch. The conventional spring-wound clutch comprises a coaxial input member on which an annular armature plate is rotatably provided along with an output member. A coil-wound spring is wound around the input member and is connected at its respective ends to the armature plate and output member. An electromagnetic coil, which is coaxially disposed in the input member with a small gap therebetween, attracts the armature plate into frictional engagement with the input member, thereby causing the coil-wound spring to be tightly wound around the input member and thus drivably engaging the input member with the output member. When the coil is de-energized, the armature plate disengages from the input member and is returned to a predetermined position due to the recoil strength of the coil wound spring. The coil-wound spring thus releases the input member from rotation relative to the output member.
During disengagement of the armature plate in an electromagnetic clutch of the above construction, the armature plate is maintained in its predetermined position only by the recoil strength of the coil-wound spring. Therefore, the armature plate tends to vibrate during and after disengagement, causing objectional noise and undesirable wear on the coil-wound spring. Moreover, the armature plate does not disengage completely parallel to the imput member, thus increasing the likelihood of the armature plate inadvertently contacting the input member. In one proposed solution to this problem, a plurality of permanent magnets are attached to the armature plate. When the electromagnetic coil is de-energized, the armature plate is returned to its predetermined position by the attractive force of the permanent magnets. The durability of the armature plate with magnets mounted in this manner is greatly diminished, however, because the impact force generated by attraction of the armature plate to the output member directly acts on the permanent magnets. Thus, there is a tendency for the permanent magnets to become dislodged from the armature plate over time. Moreover, abrasive dust particles which form due to friction between the wound spring and the output member and the armature plate and output member are attracted to the permanent magnets. If some of the dust particles settle between the cover plate covering the assembly and the armature plate, the predetermined axial gap between the armature plate and the output member is decreased. Thus, the armature plate may contact the output member without operation of the electromagnetic clutch. When this occurs, the armature plate is subject to an abnormally high rate of wear.
Other proposed solutions for the above-described problem, such as those disclosed in U.S. Pat. No. 4,194,607 issued to Yamaguchi and U.S. Pat. No. 4,262,787 issued to Takafuta et al., are fairly complicated in construction and difficult to assemble, thus lessening the likelihood of their reliable operation. Accordingly, there is a need in the art for an engagement device for an electromagnetic coil-wound clutch which overcomes the above-described deficiencies while at the same time being simple in construction and reliable in operation.